Rubber membranes are used in a variety of applications. For example, it is common in the art to employ rubber sheet material to cover rooftops, especially flat or low-sloped roofs. One type of technologically useful membrane is prepared from olefinic rubber such as EPDM rubber, which is a terpolymer of ethylene, propylene and a diene monomer. A curative is employed to crosslink the EPDM terpolymer and thereby create a durable protective membrane. Sulfur curatives are often used to cure or crosslink the rubber. Advantageously, EPDM has a fully saturated backbone, which provides resistance to ozone and oxidation.
These protective membranes must be able to withstand severe environmental conditions without the membrane failing. Depending on the geographic location, the EPDM terpolymer membranes must be able to withstand temperatures that can reach up to as high as about 90° C. for prolonged periods of time. Further, these membranes must be able to withstand long exposure to UV light and ozone without undergoing chemical degradation. Further, these membranes must withstand mechanical stresses that are experienced, as least in part, from wind uplift forces. As is known in the art, useful EPDM membranes must meet the performance standards set forth in ASTM D 4637-03 and D1418-85.
While technologically useful, these membranes have a limited life. In many situations, the service life does not exceed thirty years. Replacement of roofing membrane may include placement of new membrane over the existing membrane, or removal of the existing membrane followed by installation of the new membrane. The latter is often desirable or required.
Removal of an existing membrane creates a disposal issue. Thousands, if not millions, of square feet of EPDM-based membranes are used to cover roofs, and therefore disposal of existing membranes is not trivial. Solutions to the problem that have been proposed include recycling of the scrap EPDM for use in other applications such as fuel or as filler in new rubber products.
Entities in many industries seek to recycle EPDM rubber in order to reduce waste and increase profits by putting scrap rubber to beneficial use. In mechanical processes, EPDM rubber is recycled simply by physically cutting and grinding the same into small pieces that can be incorporated into new rubber products, essentially as filler. The particle size of the ground rubber may vary depending on the grinding methods employed. Large particles of ground rubber, which are typically used as fuel, are obtained with regular shredders or grinders, while fine rubber particles are obtained by cryogenic grinding and used as fine fillers in rubber and plastic products. Mechanically recycled rubbers are typically employed in applications where the mechanical and physical properties of the end product are not too demanding. Few waste rubber products can be mechanically recycled for use in their original applications. For example, while others have mechanically recycled scrap EPDM roofing membranes, this recycled rubber is not used in new roofing membranes because the small pieces of recycled rubber are still vulcanized and thus not able to successfully interact with the remainder of the rubber matrix of the new roofing membrane.
In order to process the ground rubber particles and form them into useful products, they are typically mixed with virgin polymers. Although ground rubber may be incorporated into virgin rubber in this manner, the ground rubber remains phase separated from the virgin material and therefore may not become incorporated into the rubber matrix upon curing of the virgin rubber.
In light of this, efforts have been made to devulcanize rubber products, including EPDM. Chemical devulcanization is an example that is particularly distinguishable from the non-devulcanizing mechanical cutting and grinding method previously stated above. U.S. Pat. No. 6,956,065 teaches the use of amines as devulcanizing agents to devulcanize EPDM rubber. U.S. Pat. No. 5,770,632 also discloses a chemical devulcanization process.
Yet another devulcanization method involves a thermo-mechanical devulcanization process. In these processes, traditional rubber processing instruments, such as open mills and twin screw extruders, subject scrap rubber to shear and heat to achieve a partial devulcanization. U.S. Pat. No. 7,342,052 discloses a thermo-mechanical method of devulcanizing scrap rubber including heating the scrap rubber, agitating the scrap rubber in a thermo-kinetic mixer, and adding an oil to the scrap rubber as it is agitated. The thermo-kinetic mixer contemplated by the patent is a double boiler vessel including a mixing chamber having a rotor to apply shear stresses to the scrap rubber.
U.S. Pat. Nos. 2,461,192 and 6,632,918 each disclose a thermo-mechanical method of devulcanizing scrap rubber including heating the scrap rubber to a temperature within a desired range and applying shear stresses to the heated scrap rubber while subjecting the rubber to increased pressures. U.S. Pat. No. 2,461,192 contemplates use of a Banbury mixer to apply the shear stresses to the scrap rubber, and U.S. Pat. No. 6,632,918 contemplates using a twin-screw extruder to apply the shear stresses to the scrap rubber.